????:1??????:????????????

2??????????

3???sn??

4???????????

5??????????

????,??????????2?11??,??????1213??,??,???,???“????”???????????,????,??“???????”???????

???????,????,??????,??????????????,????????????????????,????,???,????,????????????,??????????????(???????????????? ???????????????? ??? ????? ???????)

?????????:??????